A common method for preparing 1,1,1,2-tetrafluorochloroethane is contacting tetrachloroethylene with HF in the presence of a catalyst. While many variations on this method are useful, a preferred method is described in commonly assigned application No. 070826, filed July 7, 1987 which utilizes a gas-phase reaction of a tetrahaloethylene with HF in the presence of a selected metal salt on a high-fluorine-content alumina support, and minimizes the production of pentafluoroethane. The produced 1,1,1,2-tetrafluorochloroethane can then be hydrodehalognated to the desired 1,1,1,2-tetrafluoroethane.
GB 1,578,933 discloses and claims a process for the manufacture of tetrafluoroethanes having the formula CHF.sub.2 CHF.sub.2 and CF.sub.2 CH.sub.2 F characterized in that a haloethane having four or five fluorine atoms of formula CF.sub.2 XCFYZ is reacted with hydrogen at an elevated temperature in the presence of a hydrogenation catalyst. In the formula X is fluorine or chlorine; and when X is fluorine, Y is chlorine or fluorine; and when Y is chlorine, Z is chlorine, fluorine or hydrogen; and when Y is fluorine, Z is hydrogen; and when X is chlorine, Y is fluorine and Z is either chlorine or hydrogen. Examples of hydrognation catalysts include nickel or metals of Group VIIIa of the Periodic Table or oxides of salts thereof.
Example 7 of the above patent discloses the hydrogenation of 1,1,1,2-tetrafluorochloroethane to produce 1,1,1,2-tetrafluoroethane using 5% palladium on charcoal as the catalyst. The selectivities to 1,1,1,2-tetrafluoroethane over a temperature range of 280.degree. C. to 420.degree. C. range from 95.8% to 97.3%.
C. Gervasutti et al., Journal of Fluorine Chemistry, 19, 1-20 (1981/82) disclose the preparation of 1,1,1,2-tetrafluoromonochloroethane from isomeric mixtures of dichlorotetrafluoroethanes through selective hydrogenolysis of CF.sub.3 CCl.sub.2 F catalyzed by palladium/carbon.
The hydrodehalogenation or hydrogenolysis of 1,1,1,2-tetrafluoromonochloroethane by the above processes produces some CF.sub.3 CH.sub.3, which results in the formation of HF in the effluent. This necessitates the removal of HF from the final product. It would be desirable to be able to hydrodehalogenate 1,1,1,2-tetrafluorochloroethane with essentially quantitative selectivity thus avoiding the need for HF separation and with maximum product yield.
This invention provides a process for preparing 1,1,1,2-tetrafluoroethane useful as a refrigerant via hydrodehalogenation of 1,1,1,2-tetrafluoromonochloroethane in the presence of a catalyst consisting essentially of palladium supported on aluminum fluoride or fluorinated alumina whose selectivity is essentially quantitative.